1. Field of the Invention
The present invention relates to a rotational speed detector, and more particularly to a rotational speed detector which can perform detection of a rotational speed with a high degree of accuracy.
2. Description of Prior Art
Generally, the rotational speed of a rotating body, such as the rotor of a motor, is accurately controlled by using a rotational speed detector which detects the rotational speed and generates an output signal transmitted as a feedback signal to a rotational speed control system. A conventional speed detector attainable to the above-mentioned purpose, as illustrated in FIG. 1 of the accompanying drawings, has a disk-like permanent magnet 3 fitted onto the shaft 2 of a rotating body, such as for example the rotor of a motor 1, and a detecting head 4 positioned in the close vicinity of the disk-like permanent magnet 3 and including an iron core 41 wound with a winding coil 43 therearound, whereby the disc-like permanent magnet 3, when rotated, induces in the detecting head an electromotive force as an output signal which is detected through a detecting circuit 5.
However, the above-mentioned conventional rotational speed detector disadvantageously has a large inertial mass, since permanent magnet 3 of relatively large weight is directly fitted onto the shaft 2 of the rotor of the motor 1. Therefore, in case of, for example, a coreless motor provided with a rotor having a small inertial mass, there has been a disadvantage that the inertial mass of the magnet 3 cannot be neglected and gives a bad influence to the performance efficiency of the motor 1.
Further, another conventional rotational speed detector, as shown in FIG. 2 of the accompanying drawings, has also been used to attain the above-mentioned purpose, in which a disk plate 6 defining radial slits therein is fitted onto the shaft 2 of the rotor of the motor 1, and a light-emitting element 7 is arranged so as to oppose to a light receiving element 8 across the disk plate 6, thereby the light receiving element 8 receives intermittent light beam through the rotating disk plate 6 from the light-emitting element 7 energized by an electrical power supply 9, and produces an output which is then amplified through an amplifier circuit 10 and detected by a detecting circuit 11.
However, such a conventional rotational speed detector has to have the light-emitting element, the electrical power supply, the light receiving element, the amplifier circuit and etc., and therefore, exhibits a very complex arrangement, which inevitably causes the rotational speed detector to be very expensive.